In connection with propagation of a device for recording and reproducing digital image data such as a digital VTR, DVD (digital versatile disc) or the like, a large number of digital motion pictures that can be reproduced by such a device as described above have been supplied. Furthermore, various digital motion pictures are being distributed through digital television broadcast through the Internet, a broadcast satellite, a communication satellite, etc., and users are allowed to use high-quality digital motion pictures.
With respect to digital motion pictures, high-quality copies thereof can be easily made at the digital signal level. Therefore, when neither copy inhibition nor copy control is carried out on these digital motion pictures, there is a risk that they are copied without restriction. Accordingly, in order to prevent unjust copying of digital motion pictures or control the number of generations of copying by regular users, there has been considered a method of adding copy controlling information to digital motion pictures and preventing unjust copying or restricting copying by using the additional information concerned.
A digital watermarking (electronic watermark) technique is known as a technique of superposing another different additional information on digital motion pictures as described above. According to the digital watermarking technique, various information such as identification information on the owner of copyright of a content (digitalized voice, music, motion picture, still picture or the like) and users of the content, right information of the copyright owner, the using condition of the content, secrete information needed to use the content, the information such as the copy controlling information described above, etc. (hereinafter referred to as “watermarking information”) is embedded in the content concerned, and copyright protection containing control of use and copy control is afterward carried out by detecting the watermarking information from the content as occasion demands or secondary use is promoted.
Various method have been proposed as a digital watermarking system, and a system using a spread spectrum technique is known as one of these digital watermarking systems. According to this system, watermark information is embedded in digital motion pictures according to the following procedure.
[Step E1]
An image signal is multiplied by PN (Pseudorandom Noise) sequence to carry out spread spectrum modulation.
[Step E2]
The image signal that has been subjected to spread spectrum modulation is subjected to frequency-transformation (for example, DCT transformation). [Step E3]
Watermarking information is embedded by changing the value of a specific frequency component.
[Step E4]
Inverse frequency transformation (for example, IDCT transformation) is carried out.
[Step E5]
Inverse spread spectrum modulation is carried out (the image signal is multiplied by the same PN sequence as step E1).
Detection of the watermarking information from the digital motion picture in which the watermarking information is embedded as described above is carried out according to the following procedure.
[Step D1]
An image signal is multiplied by PN (Pseudorandom Noise) sequence (the same PN sequence as step E1) to carry out spread spectrum modulation.
[Step D2]
The image signal that has been subjected to the spread spectrum modulation is subjected to frequency transformation (for example, DCT transformation).
[Step D3]
The value of a specific frequency component is noted and watermarking information embedded is extracted.
A technique for detecting watermarking information through the correlation (mutual correlation) between an extraction signal achieved by extracting an input image signal and an image having a watermarking embedded therein has been proposed as a method for detecting digital watermarking embedded in a motion picture in which an image signal is embedded according to watermarking information. For example, this technique is disclosed in Patent Document 1 (claim 2, FIG. 7 of JP-A-2002-325233) and Patent Document 2 (claim 1, FIG. 1 of JP-A-2002-218404).
When digital watermarking is applied to prevent unauthorized use, digital copyrighted works are required to have such a property (robustness) that watermarking information is not lost or falsified by various kinds of operations or intentional attacks which are assumed to be normally conducted on digital copyrighted works. Cut-out of images, scaling (enlargement/reduction), etc. may be considered as an attack of disenabling detection of watermarking information from a digital image in which the watermarking information is embedded.
According to the conventional technique, when an image suffering such an attack is input, the processing of estimating PN sequence used in step E1 when watermarking information is embedded is first carried out when the watermarking information is detected, and synchronization of the PN sequence is restored. Thereafter, the processing of the steps D1 to D3 is carried out to extract the embedded watermarking information. When the synchronization of the PN sequence is restored from only the image signal, it is required to carry out such a searching operation that the processing is tried by using plural candidates and a candidate for which the detection can be carried out is adopted as the watermarking information. Therefore, this method has a problem that the calculation amount and the circuit scale are increased. Furthermore, with respect to an image suffering an attack, the watermarking information of the image is weakened, and thus even when cut-out or scaling of the image is found out and the detection adapted to the cut-out or scaling of the image is carried out, the watermarking information cannot be detected.